Sleep is conceived as a unitary phenomenon in the sense that both its phases (slow-wave sleep and paradoxical sleep) start during the same phylogenetic steps, in reptiles, and their mechanisms, though distinct, are functionally coupled in the vertebrate series. Slow-wave sleep, essentially a restoring phenomenon, seems to be required by the specific and associative thalamocortical circuits and triggered off by the subcortical hypnogenic structures, by virtue of low-frequency tranquillizing afferent volleys received from sense organs. Paradoxical sleep is appreciated as a special hypnic behavior displayed against the slow-wave sleep tonic background, with an ancestral role of "guardian" of slow-wave sleep, and with present complex functions, many of which not yet elucidated.